It is known, in such a system, to equip each outlying station with a video transmitter and a video receiver in addition to the usual telephone apparatus, these instruments being linked by audio and video lines with the common central office. Each station also includes a source of synchronizing pulses which controls the sweep circuits of its own video transmitter as well as those of a remote receiver temporarily connected thereto by way of the central office. The synchronizing pulses are generated by individual crystal-stabilized oscillators operating independently of one another; such oscillators are relatively expensive and their duplication at each of the several stations weighs heavily in the overall cost of the system.